The present invention relates to fiber optic transmission systems and more particularly to optical links and networks having a very high capacity and covering very long distances. By optical links with a very high capacity is meant transmission systems providing a bit rate greater than 10 Gbit/s. By transmission systems covering very long distances is meant systems covering propagation distances of the order of 5000 km or more.
At present obtaining very high capacities on long systems is not possible, apart from N*2.5 Gbit/s wavelength division multiplex systems, even though papers have been published reporting laboratory results that are as yet incompatible with the requirements of real systems.
At present, RZ (return to zero) pulse transmission and NRZ (no return to zero) pulse transmission are widely used in long-haul fiber optic transmission systems. One problem for this type of system is that the signal/noise ratio increases as the number of repeaters in the system increases, due in particular to amplified spontaneous emission (ASE) noise.
In the case of soliton signal transmission systems, to reduce the amplified spontaneous emission noise, and thereby to increase the signal/noise ratio, it has been proposed, for example in EP-A-0 576 208, to use sliding guiding filter systems. This solution is based on the particular nature of solitons and their capacity for selfregeneration. In other words, soliton signals track the sliding of the filters, whereas the amplified spontaneous emission noise is filtered out.
This solution also applies to types of signals other than soliton signals. However, because it relies on self-phase-modulation of the signals, it is difficult to implement for wavelength division multiplex transmission systems, because of crossed phase modulation between channels. The passage of the signals through the sliding filters implies a high level of self-phase-modulation, which goes hand in hand with a high level of crossed phase modulation. To obtain results with this type of solution in a wavelength division multiplex transmission system it would be necessary to separate the various channels well beyond the band of a few nanometers available for the signals.
The transmission of signals in optical systems is also limited by non-linear effects, such as the Kerr effect, the Brillouin effect, the Raman effect and four-wave mixing. G. P. Agrawal, xe2x80x9cNonlinear Fiber Opticsxe2x80x9d, Academic Press, 1980 describes these non-linear effects. They depend on the level of noise in the optical fibers of the transmission system.
The invention proposes a solution to the problem of increasing noise, and in particular of amplified spontaneous emission noise in a fiber optic transmission system. It significantly improves the quality factor of transmission systems, especially systems of very high capacity and covering long distances. The invention eliminates most of the noise at the wavelengths of the signals transmitted and makes possible xe2x80x9clinearxe2x80x9d transmission that is not limited by the noise level; it also makes possible nonlinear transmission limited by the effects of the noise. Some embodiments of the invention reduce the timing jitter of the signals.
To be more precise, the invention proposes a noise-reducing device for a fiber optic transmission system, said device including
first means for filtering noise outside the range of wavelengths of the signals transmitted,
means for shifting the wavelength of the signals transmitted, and
second means for filtering the transmitted signals that have undergone wavelength shifting.
The device advantageously includes second wavelength shifting means for returning the signals that have undergone the second filtration to their initial wavelength.
In one embodiment the wavelength shifting means include means for widening the spectrum of the signals.
The wavelength shifting means preferably include optical phase conjugation means.
In one embodiment the filtration means include a Bragg filter.
The invention also provides a fiber optic transmission system including at least one such device.
The invention further provides a method of reducing noise in a fiber optic transmission system, said method comprising the steps of
filtering noise outside the range of wavelength of the signals transmitted,
shifting the wavelength of the signals transmitted, and
filtering the signals transmitted that have undergone wavelength shifting.
In one embodiment the method further includes a second wavelength shifting step for returning the signals that have undergone the second filtration step to their initial wavelength.
The wavelength shifting step advantageously includes widening the spectrum of the signal.
The wavelength shifting step can also include conjugation of the phase of the signal.